Millikan's Oil Experiment

My question is that why did Millikan allow the oil drop to fall in terminal velocity and thus solving for the radius of the oil drop to find its mass eventually and not using a microscope to measure the oil drop's radius?

This is one part of my tutorial question and I really have no idea though it sounds simple. Appreciate any inputs!

My question is that why did Millikan allow the oil drop to fall in terminal velocity and thus solving for the radius of the oil drop to find its mass eventually and not using a microscope to measure the oil drop's radius?

Two options :

1) Millikan did not have a microscope that could do the job properly
2) He wanted to avoid the inaccuracies that arise when using such device. Therminal velocity is an "universal" concept that is properly described by newtonian physics. Using this concept does not introduce additional inaccuracies due to any kind of experimental measurement.